Since moving to Office 365 for some users, I came across a problem that didn't really appear to be documented in the Hybird Process.

This issue was Dynamic Distribution Lists.

If Dynamic Distribution Lists in your company, like ours, had the only Attribute, "MailboxUsers" your Office 365 users wouldn't get the email sent to that list, even though other settings on that list indicated they would.

To allow Office 365 users to receive messages from users who are on premise, you need to include the attribute "MailUsers" in the dynamic list.

Step 2: Check your list

Once you've looked over your list, and see what the current included recipients look like, you can then move onto the next step which sets the Distribution list to add the MailUsers in the IncludedRecipients Attribute.